True blacks do occur.
the genetics of the black coat
Black horses have at least one E+ allele at the extension locus (i.e. They are of genotype E+E+, E+e or E+ea). This dominant allele causes the production of the black eumelanin pigment.
The A or agouti locus controls the distribution of black pigment in horses with at least one E+ allele: whether it occurs evenly throughout the body, as in true black horses, or only in certain parts, as in bays and browns.
The recessive allele Aa of the agouti locus has no effect on eumelanin production. Horses of genotype AaAa therefore have a base color of black. It is the Aa allele that is rare in some breeds. Breeders interested in producing black foals would ideally have horses of genotype AaAa E+E+, I.e. True-breeding blacks.
The coats of some black horses fade Ė or rather redden - in the sun. The genetic basis of so called fading and non-fading black isnít currently known. However the phenomenon is well known and not confined to horses, occurring in people and cats, and probably in other animals too.
Since red (chestnut) horses are true-breeding for red it is easy to breed red foals. It is more difficult to breed black horses, since they may be of genotype E+E+ or they may be heterozygous. Breeding together heterozygous blacks may produce chestnut foals.